This invention relates generally to magnetic resource imaging (MRI), and more particularly the invention relates to magnetic resonance angiography (MRA) using an oscillating dual-equilibrium steady state free precession process.
Magnetic resonance angiography (MRA) is a clinically relevant non-invasive alternative to traditional X-ray angiography. A number of approaches have been used to isolate signal from vessels. Inflow-based methods, such as time-of-flight and spin-tagging, waste significant scan time while waiting for inflow to occur and may suffer from reduced signal in distal portions of the artery being imaged. Flow-independent techniques make use of the inherent T1 and T2 relaxation times of arterial and venous blood to generate the desired contrast. However, these techniques suffer from incomplete suppression of background signal and may require careful selection of numerous scan parameters for ideal performance.
Phase contrast angiography (PCA) makes use of the phase accrual of moving transverse spins during the application of a gradient. This technique can suppress background signal very well; however, the resulting blood signal strength is proportional to flow in the direction of flow sensitivity. As a result, three acquisitions are often required to get uniform signal from the extent of the vessel. Care must also be taken to match the velocity sensitivity of the sequence to the flow rate in the vessel of interest. Finally, because signal phase is proportional to velocity, thick-slab images suffer from signal loss in pixels where multiple vessels overlap.
T1-shortening contrast agents, primarily gadolinium compounds, have also been used successfully to produce angiographic images. These techniques boost blood signal while keeping background signal from other spins constant; this background signal can reduce image contrast. Moreover, several doses of contrast agent are often required in order to produce the necessary contrast, and acquisitions must be carefully timed in order to isolate arterial signal.
The invention is a new angiographic process, which is denoted oscillating dual-equilibrium steady state angiography (ODESSA). This method incorporates velocity-dependent phase shifts into a modified steady state free precession (SSFP) process in order to select a blood vessel signal. Refocused SSFP pulse sequences, which refocus all magnetization over the RF repetition interval TR, provide a means of rapid imaging while maintaining high signal. Until recently, these methods have been largely ignored for imaging applications because of their sensitivity to off-resonant precession. Recently, high-performance imaging systems have allowed refocused SSFP sequences with very short TRs, which minimize these unwanted artifacts.
More particularly, the invention utilizes short-TR refocused SSFP sequences to rapidly produce images with angiographic contrast. In order to isolate signal from blood, an oscillating steady state is generated for flowing material through the periodic addition of a bipolar flow-encoding pulse, such as every second TR, for example. Static tissue is unaffected by this bipolar pulse, and thus approaches a single steady state. Addition of adjacent echoes produces anatomic contrast similar to that of standard SSFP sequences. Subtraction of adjacent echoes results in suppression of signal from stationary tissue, while the signal from flowing blood retains the high SNR expected of SSFP sequences. Furthermore, the subtracted signal from flowing blood is uniform in magnitude and phase over a wide range of velocities. Therefore, scans with multiple directions of velocity sensitivity may not be necessary, and thick-slab projection imaging is possible. Contrast between arterial and venous blood can be adjusted through manipulation of the RF tip angle. The technique can also be used in conjunction with a T1-shortening contrast agent to further enhance SNR.